Project Summary/Abstract Although current treatments for opioid use disorder are associated with decreased opioid use and reduced overdose risk, approximately half of patients do not adequately respond to treatment. Heightened reactivity to stress is a robust predictor of relapse in opioid use disorder and does not improve with medication-assisted treatment. This may be particularly important for women with opioid use disorder, who are more likely than men to have co-occurring depressive and anxiety disorders and are more likely to report using opioid to cope with negative affect. One mechanism that may help to explain this gender difference is the impact of gonadal sex hormones, including estrogen and progesterone, on responses to stress. Fluctuation of gonadal sex hormones across the menstrual cycle impacts risk for substance use, relapse, and stress-related negative outcomes in women. Improved understanding of sex differences in stress reactivity will help to identify targeted personalized treatments for improving outcomes in opioid use disorder. However, little is known about the impact of gonadal sex hormones on stress reactivity in opioid use disorder. The proposed supplement will complement and enhance an ongoing study examining behavioral treatments for reducing stress reactivity opioid use disorder by examining the role of gonadal sex hormones (estradiol, progesterone and testosterone) on stress reactivity. The proposed supplement consists of two aims (1) to examine whether gonadal hormones are associated with stress reactivity in adults with opioid use disorder, (2) to determine whether gonadal hormones are associated with response to behavioral strategies to reduce stress reactivity. The results from this study will provide important information about a potential mechanism underlying the maintenance of harmful opioid use, particularly among women. Ultimately, this information can be used to develop novel behavioral and pharmacologic treatment strategies for women with substance use disorders.